This invention relates to solid state device packaging and specifically to an encapsulating material for a LED. It also relates to a method of encapsulating a solid state device, particularly a LED.
Light transmissive resins for encapsulating semiconductor devices have heavily relied on blends of bisphenol-A epoxy and aliphatic anhydride curing agents. As described in U.S. Pat. No. 4,178,274, to Denk et al., xe2x80x9cone disadvantage of epoxy casting resin compounds based on bisphenol A/acid anhydride, which harden fast through the use of known accelerators such as tertiary amines, imidazoles or boron trifluoride complexes, is their poor thermal aging stability. The materials used heretofore become discolored in extended storage at temperatures above 80xc2x0 C., through yellow to brown, whereby their light transmittancy decreases considerably.xe2x80x9d Furthermore, because of the aromatic character of bisphenol-A based epoxy resins, these encapsulants would be less stable to ultraviolet radiation. Therefore, these materials may tend to degrade on extended exposure to light having an ultraviolet component. Such degradation can lead to discoloration of the encapsulant, and accordingly reduced light transmittance.
To circumvent these issues, Denk et al. describe casting resin compositions for the sealing of opto-electronic components. These resins comprise (i) at least one cycloaliphatic epoxy having at least two oxirane rings, (ii) a carbonic acid anhydride, (iii) zinc octoate, and (iv) a solvent selected from the group consisting of a low molecular weight polyol, a low molecular weight ester and mixtures thereof. The inventors of the present invention have calculated the weight of epoxy resin to be at most 46%.
Wada et al. in U.S. Pat. No. 5,145,889 describe a composition comprising (i) 100 parts by weight of an epoxy resin, (ii) 70 to 140 parts by weight of a curing agent including an acid anhydride, (iii) 0.5 to 4.0 parts by weight of a curing accelerator including an onium or diazabicycloalkene salt, (iv) 0.5 to 5.0 parts by weight of a phosphorus triphosphite, and (v) 0.5 to 5.0 parts by weight of a silane coupling agent. The inventors of the present application have calculated the epoxy resin of Wada to be at most 58% epoxy resin by weight.
The inventors of the present invention have found that addition of such high levels of curing agents can lead to color formation in thermally cured resins, thereby reducing the overall transmittance of the light emitting diode. Because the typical process for the production of a LED involves casting of an;uncured epoxy resin around the LED device followed by thermally curing the epoxy, such thermal discoloration is undesirable. Whereas UV curable epoxy resins can be formulated to provide compositions with levels of epoxy above 76%, such materials are not desirable for processes involving thermal cure. Furthermore, materials such as diaryliodonium salts are known to produce high colors upon cure.
Uram et al. in U.S. Pat. No. 4,454,201 describe a multi-layered transparency composite for use in military and industrial hardware, such as windows for military helicopters. The transparency composite has a blend of (i) from about 80 to about 100 parts by weight of an epoxy resin, (ii) from about 5 to about 30 parts by weight of a boroxine, such as trimethoxyboroxine, (iii) from about 1 part to about 40 parts by weight of an organic phosphorus compound, that is contiguously contacting at least one layer of a composition resistant to moisture permeation, which comprises (iv) 100 parts by weight of a mercaptan resin, (v) 40 to about 250 parts by weight of an epoxy resin, and (vi) from about 0.5 parts by weight to about 4.0 parts by weight of a silane coupling agent.
Morris et al. in U.S. Pat. No. 4,336,367 describe an adhesive composition for clandestinely attaching ordnance devices to the bottom of ships. This composition comprises a mixture of a (i) 3,4-epoxycyclohexylalkyl 3,4-epoxycyclohexanecarboxylate, exo-exo bis(2,3-epoxycyclopentyl) ether, endo-exo bis(2,3-epoxycyclopentyl) ether and (ii) a trialkoxyboroxine. The compositions are developed for underwater adhesive applications for use in naval warfare. There is no disclosure given for the use of such compositions for encapsulation of solid-state devices such as LEDs.
In accordance with one aspect of the present invention, there is provided a packaged solid state device comprising a package, a chip, and an encapsulant comprising an epoxy resin, and a boron-containing catalyst that is essentially free of halogen.
In accordance with another aspect of the present invention, there is provided a LED device comprising a package, a LED chip, and an encapsulant comprising a cycloaliphatic epoxy resin, and a boroxine catalyst that is essentially free of halogen.
In accordance with another aspect of the present invention, there is provided a method of encapsulating a solid state device comprising placing a solid state device into a package, and providing an encapsulant comprising an epoxy resin and a boron containing catalyst that is essentially free of halogen.
In accordance with yet another aspect of the present invention, there is provided a LED device comprising a package, a LED chip, and an encapsulant comprising a cycloaliphatic epoxy resin from about 76% to about 99% by weight of the encapsulant, a boroxine catalyst from about 1% to about 24% by weight of the encapsulant, wherein the catalyst is essentially free of halogen, and a thermal stabilizer.
In accordance with another aspect of the present invention, there is provided a LED device comprising a package, a LED chip; and an encapsulant comprising a cycloaliphatic epoxy resin from about 76% to about 99% by weight of the encapsulant; and a curing agent or catalyst, from about 1% to about 24% by weight of the encapsulant, wherein the curing agent or catalyst is essentially free of halogen.